Why Some Giant Planets Don't Appear To Have Water

Artist's impression of the ten gas giants whose atmospheres were investigated. Clockwise from top left: WASP-12b, WASP-6b, WASP-31b, WASP-39b, HD 189733b, HAT-P-12b, WASP-17b, WASP-19b, HAT-P-1b and HD 209458b. ESA/Hubble/NASA

The largest study ever conducted of the atmospheres of planets circling other stars has revealed how different they can be and explained why some appear to contain so little water – and it seems to be that clouds are to blame, or a lack thereof.

The findings help us understand the circumstances that influence giant planets' evolution, and confirm our suspicion that water is common even on planets so hot it can only exist as a gas.

Now that the excitement of merely finding planets beyond the Solar System has faded, astronomers are keen to learn more about these worlds than just their mass and orbital distance. Sadly, in most cases there is nothing we can discover with our current technology.

When a planet passes between its parent star and Earth the star’s light is filtered through the atmosphere of the planet. Most of the planets we know about are either not positioned suitably for this, or are too distant to allow us to collect useful data. However, in a few cases, enough light can be collected for astronomers to see which wavelengths of light have been absorbed in passing through the planetary atmosphere.

Each type of molecule has a unique signature of absorbed wavelengths, so the light that reaches us reveals the atmospheric content. In Nature a team led by Dr. David Sing of the University of Exeter have conducted a study of ten planets' atmospheres, the most ever done in such a way, allowing comparisons between them.

A further step forward was the use of both the Hubble and Spitzer Space Telescopes for a single study. Hubble collects visible and nearby wavelengths while Spitzer operates in the infrared. “This is the first time we've had sufficient wavelength coverage to be able to compare multiple features from one planet to another," said Sing in a statement. “We found the planetary atmospheres to be much more diverse than we expected."

All the planets involved are so-called “hot Jupiters”, gas giants that orbit very close to their parent stars.

Among the ten worlds studied were two planets that have made headlines before. HD 209458b, located 150 light-years away in the Pegasus constellation, was the first extrasolar planet found that transits its parent star, as seen from Earth, and the first proven to have an atmosphere. Just last month HD 189733b, which is a relatively close 63 light-years away, was reported to have wind speeds of thousands of kilometers an hour.

Both these two, along with a third planet, Wasp-12b, were reported last year as being surprisingly dry, with between a tenth and a thousandth of their expected concentrations of atmospheric water. This was puzzling because some planets in this class are rich in water.

When comparing the ten planets, the researchers realized that those with plenty of detectable water also lacked clouds that would interfere with the light reaching Earth. On the other hand, the low-water planets were cloudy, leading the authors to conclude that the clouds are interfering with our capacity to identify the water in those atmospheres.

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